Process for chromium plating



1959 J. E. STARECK ETAL 2,916,424

PROCESS FOR CHROMIUM PLATING Filed NOV. 7, 1957 FIG. l.

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a ma zoo 300 400 :00 600 7/ 6h #vvzwroes A5555 5 Jr/YRECK E0614 4/- .SEYB, F. ANDY ALBERT JOHNSON WILL/AM fl KOWAN ATTORNEY United States Patent O PROCESS FOR CHROMIUM PLATING Jesse E. Stareck, Westfield, NJ., and Edgar J. Seyb, Jr.,

Andy Albert Johnson, and William H. Rowan, Oak Park, Mich., assignors to Metal and Thermit Corporation, a corporation of New Jersey I Application November 7, 1957, Serial No. 695,146

6 Claims. (Cl. 204-51) The present invention relates to a novel process for electrodepositing chromium, and more particularly, to a 1 process for electrodepositing bright crack-free chromium plate.

Chromium is widely used as a decorative and protective surface finish. Decorative chromium plate is usually .of too small a thickness, in order of 0.00001 inch to be of .substantial value for protecting the basis metal. When deposited in greater thicknesses, the plate contains a network of hair-like cracks extending through the chromium to the basis metal, destroying the plates protective value.

[To coat metals with a corrosion resistant, decorative electrodeposit, copper and/ or nickel are deposited on the basis metal to provide effective corrosion resistance, followed by deposition of the usual thin decorative chromium finish. Thick deposits of chromium are deposited directly on the basis metal or over a copper and/or nickel undercoat to provide fairly effective protection against corrosion and wear. These thick deposits are usually in the order of at least 0.002 inch. When chromium is electrodeposited to a thickness of between 0.00001 inch and 0.00002 inch (usually closer to 0.00002 inch), the de- 1 used for decorative applications. Recently a process has become available for electrodepositing dull crack-free chromium in thicknesses greater than 0.00002 inch (see US. Patents No. 2,686,756, No. 2,787,588, and No. 2,787,589).

Chromium deposits in the order of 0.00005 inch, 0.0001 inch and greater, which are bright and crack-free as plated, are desirable for providing decorative chromium plates having good corrosion resistance. It has been found that for a given thickness of deposit; e.g., 0.00005 inch, 0.0001 inch and greater, the corrosion resistance of crack-free deposits is far superior to the corrosion resistance of deposits having a crack pattern, even including those deposits of sufiicient thickness that the cracks are healed. The art has not known how toobtain thick, bright crack-free chromium deposits and it appeared that it would be impossible to obtain them. The known conditions for obtaining bright decorative deposits are, on the whole, the opposite of the conditions necessary to obtain crack-free deposits.

The term bright in describing bright crack-free chromium electrodeposits, as used herein, is limited to surfaces which have suificient reflectivity to render a well defined and recognizable image. The bright range of deposits is divided into two subdivisions-deposits referred to as mirror-bright and deposits referred to as bright.

The mirror-bright deposits are those which have the re- U 2,916,424 Patented Dec. 8, 1959 2 flectivity ordinarily associated witha good mirror. The bright deposits have a lesser degree of reflectivity than a good mirror, but suflicient reflectivity to render a well defined and recognizable image. These bright deposits are sufliciently bright as plated to be acceptable decorative finishes. Their degree of brightness may be improved by a bufiing operation to achieve mirror brightness. The

term bright crack-free chromium encompasses those deposits which are mirror-bright and bright, as defined herein. j

It is an object of the present inventionto provide a process for electroplating bright crack-free chromium deposits to a thickness of at least 0.00005 inch.

It is another object of the invention to provide a process for electrodepositing'bright chromium having improved corrosion resistance.

It is still another object of the invention to provide a process for plating crack-free chromium surfaces of mirror-brightness to a thickness of at least 0.00005 inch.

Other objects and advantages will become apparent from the following description and drawings. I,

We have discovered that by controlling the several variables of the chromium plating process within relativelynarrow well-defined limits, bright crack-free depositsof chromium can be obtained in a thickness of at least 0.00005 inch'using'a plating bath of chromic acid and a mixture of sulfate and silicofluoride catalysts. Catalyst ions in relatively high concentrations promote cracking. Nonetheless, relatively large concentrations of catalystions are required to obtain bright electrodepo sits. We have discovered that bright crack-free chromium can be obtained when utilizing chromic acid baths containing between about 1.2 g./1. and about 6.3 g./l. of total catalyst ions. 'The amount of catalyst ion that may be present in an operative bath isrelated to the CrQ concentration. It is specified as the ratio of the weight of CrO to the. weight of all the catalyst ions ,(herein referred to as Ratio). The range of Ratios within which it is possible to obtain bright crack-free chromium plating is between :1 to 150: 1, dependent upon the CrOQ concentration and the temperature] In conjunction with the close control of catalyst concentration, we have found that the temperature must be maintained above 112 F. and below F. With temperatures above about 119 F. and below 140 F., it is possible to obtain bright crackfree chromium electrodeposits utilizing any Ratio between 80:1 and :1. As the temperature is lowered the tendency to crack increasesmarkedly. To compensate for this increased tendency to crack at lower plating temperatures, baths having lower catalyst concentrations must be employed. Baths with very low catalyst concentrations, as noted by Ratios as high as 150:1, may be utilized at temperatures as low as 112 F. As the temperature is raised, the tendency to electrodeposit bright chromium decreases. Dull deposits are obtained at high temperatures. The upper temperature limit for obtaining bright chromium for the conditions specified is 140 F. The temperature and Ratios at which bright crack-free chromium may be obtained are also related to the CrO concentration of the bath. Baths containing from about g./l. to about 540 g./l. are utilized. The tendency for the electrodeposition of chromium in a less bright condition increases as the CrO concentration is increased. It is difficult to obtain mirror bright deposits with baths having CrO concentrations in the high portion of the range. Such baths may be utilized at lower temperatures. Baths with .CrO concentrations in the lower portion of the range yield mirror bright deposits. The larger portion of theCrO; concentration range may be utilized at all temperatures above 119 F. and below 140 F. The overall relationship of temperature, CrO concentration, and Ratio is defined by the area enclosed 3 by the curves in Figure 1. The catalyst is a mixture sulfate ions and silicofluoride ions. Of the total catalyst present, a minimum of 20% is sulfate and preferable at least 35% should be sulfate. The sulfate should not be-more than 85% of the total catalyst and preferably not more than 75% As shown in Figure 1, bright crack-free chromium can be lectrodeposited at temperatures between 112 F. and 11-9 F., by utilizing a high Ratio. At 115 F., a Ratio of about 115:1 is the limiting Ratio, whereas the limiting Ratio at 112 F. is 150:1.

Many combinations of the three variables result in electr'odeposit's of bright crack-free chromium appreciably thicker than 0.00005 inch. These thicknesses may often be in the order of 0.0001 inch to 0.0002 inch, and in cases as thick as 0.001 inch or more. As a general rule, for agiven set of conditions, as the deposits get thicker, there is an increased tendency to crack and/or to obtain deposits of decreasing brightness.

Electrodeposition of bright crack-free chromium between the temperaturesof 117' F. and 135 F. is preferred. Within this range of temperatures'it is preferred to utilize CrO concentrations between about 250 g./l. and about 425 g./l.; the relationship between the three variables being that defined by the enclosed areas of the curve in Fig. 2. In the large enclosed area ABC, all Ratios between 80:1 and 150:1 may be used. In the area segment ABD the preferred range of Ratios is progressively more narrow with the limiting preferred range at line ADB being 115:1 to 150:1.

Maximum thicknesses of bright crack-free chromium are deposited when plating within the rectangular area ABCD of Fig. 1, at Ratios between about 105:1 and 135:1, utilizing baths having a CrO concentration between about 275 g./l. and 400 g./l. and plating at temperatures between about 125 F. and 138 F.

Although it is possible by accurate analytical control, or by the use of self-regulating bath compositions, to control the Ratio within close limits during the plating process, it is recognized that many installations may not be able to maintain the desired close control for lesser or longer periods of time. When the temperature and CrO concentration are maintained within the area defined by the closed curve AEFG in Fig. 1, it is possible to deposit bright crack-free chromium at all Ratios between 80:1 and 150:1.

Chloride ion in the bath promotes the formation of cracks. Baths containing more than 0.05 g./l. of chloride ion cannot be used to electrodeposit bright crackfree chromium to a thickness of 0.00005 inch. Less than this amount may be tolerated when electrodeposition is carried out, for a given temperature and C'rO concentration, at a Ratio selected from the high end of those allowable under the conditions specified. If it is desired to operate at a low Ratio (high catalyst ion content) selected from the allowable range, it is necessary to compensate for the presence of chloride by subtracting from the allowable catalyst ion content about fourteen times the amount of chloride ion present. When less than 0.005 g./l. of chloride is present, it may be ignored. It is preferable to operate with baths containing not more than 0.02 g./l. of chloride ion.

Chromium plating baths are frequently designated as chromic acid baths. Herein the chromic acid content of the bath is referred to asCrO (more accurately designated chromic anhydride). The bath may be made up by supplying CrO in the form of chromic anhydride or in the form of compounds containing cations which do not adversely affect the bath characteristics. Such compounds include the chromates, dichromates, and polychromates of potassium, sodium, magnesium and calci- The CrO may also be added in the form of chromic acid 'and/'or dichromic acid in solution. Where alkali metal cations are present the bath should not be neutr'ali'ze'd in excess of 80% to the dichromate end-point.

Sulfate ions may be added in the form of sulfuric acid,- or as sulfate salt(s) with a cation that does not ad-' versely affect bath characteristics, such as potassium, sodium, calcium, strontium, magnesium, chromium, etc. silicofluoride ions may be added in the form of fluosilicic acid, or as silicofluoride salt(s) with a cation that does not adversely affect bath characteristics, such as potassium, sodium, magnesium, chromium, etc. The amount of sulfate and of silicofluoride added to the bath, and maintained in the bath, must be such that their sum results in a Ratio that conforms to the limits set herein. The desired catalyst ion concentration may be attained by adding soluble salts and controlling concentration by analysis. Baths having suitable sulfate ion and/ or silicofluoride ion characteristics may be made-up by adding to the bath a sulfate compound and/or a silicofluoride compound in excess of their solubility in the bath, where the compounds selected have solubility characteristics in the bath so that the resultant total ion concentration is within the limits specified herein. For sulfate control, this could be varied somewhat by adding, with a relatively insoluble sulfate compound, e.g. strontium sulfate, present in excess of its solubility in the bath, a more soluble sulfate compound such as sulfuric acid or sodium and/or potassium sulfate to adjust the sulfate concentration upward to that desired, within the limits specified herein. Baths having lower sulfate concentrations may be prepared by adding the sulfate in the form of a relatively insoluble compound; e.g., strontium sulfate, in excess of its solubility in the bath and also adding a more soluble compound having a common cation with the sulfate compound and having an anion that is non-catalytic and that does not adversely alfect the bath characteristics; e.g. strontium chromate, strontium carbonate, etc. In a similar manner, the silicofluoride ion content may be varied and still controlled by the proper choice of compounds. Potassium silicofluoride is relatively insoluble and when present 'in excess of its solubility may result in a bath having a proper ratio when taken in conjunction with sulfate ion. Where a lower silicofluoride ion concentration is desired, potassium silicofluoride should be used in conjunction with another more soluble potassium compound, such as potassium chromate or dichromate, potassium carbonate, etc.

It is preferred to utilize chromic acid baths prepared and maintained from compositions in which the silico fluoride is added as K SiF in an amount in excess of its solubility in the bath, the silicofluoride concentration being further lowered by the inclusion of a soluble potassium compound, such as potassium dichromate. 'Ihe sulfate is preferably added as SrSO, in excess of its solubility in the bath. If desired or required by the conditions for obtaining bright crack-free chromium, the sulfate ion concentration may be lowered by the addition of a soluble strontium compound such as strontium chromate.

Bright crack-free electrodeposits are obtained under the conditions of this invention using suitable current densities. The current densities vary between 0.5 a.s.i. (amperes per square inch) and 6 a.s.i., depending primarily on the plating temperature. These current densities are conventional and are described in Chromium Plating by Morisset et al., published by Robert Draper Ltd., 1954, pages 351-363 and particularly page 354.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illus' trative examples are given.

A standard Hull cell was used to determine the thickness to which bright chromium plate can be electrodeposited without cracking. The cell is a box with an anode perpendicular to the sides and an inclined cathode. The current density on the cathode varies inversely with the distance from the near end where the current density is highest and the electrodeposit is thickest. The chromium plated cathode was examined for cracks and for brightness, and a thickness measurement made at the region where the crack pattern begins and/or where the plate is no longer bright. Bright crack-free chromium was electrodeposited from all the baths listed below ,(defined by their CrO sulfate ion and silicofiuoride ion concentrations) at the specified conditions to the thickness, in all cases, of at least 0.00005 inch.

Plating No. Or;, S0 g.ll. 81F Temperg./l. al ure.

300 1. 00 2. 00 120 300 1. 25 1. 25 120 300 0. 83 1. 66 120 300 1. 00 1. 00 120 400 2. 00 2. 00 120 400 1. 66 1. 06 120 400 1. 11 2. 22 120 400 1. 34 1. 34 120 200 0. 84 0. 84 135 200 0. 56 1. 12 135 300 1. 50 1. 50 135 300 l. 00 2. 00 135 300 1. 25 1. 25 135 300 0. 83 1. 67 135 300 1. 00 1. 00 135 300 0. 66 1. 33 135 400 2. 00 2. 00 135 400 1. 30 2. 60 135 400 1. 67 1. 67 135 400 1. 11 2. 22 135 400 1. 34 l. 34 135 400 0. 89 1. 78 135 300 0. 7 l. 3 115 500 1. 17 2. 16 114 300 0. 7 1. 3 139 500 1. 17 2. 16 125 The baths for example Nos. 27-31 were prepared from baths whose CrO concentrations was supplied by CrO and large additions of K Cr O- (although in no case equal to the amount of CrO; added per se). K siF and SrSO were present in excess of their solubilities in the bath. The baths also contained additions of between 2.8 g./l. and 17.2 g./l. of SrCrO The analyses of the baths at equilibrium, at the specified temperatures Example No. 32 was prepared in the manner of Nos. 27-31 except that there was no addition of strontium chromate and of the total amount of CrO and x cr o 71.5% by weight was added as K Cr O 0:0,, 81F Temper- No. g./l. S04, g.ll. g./l. agurre,

We have discovered that other materials which may be found in the bath affect the ability to plate bright crack-free chromium. Certain materials which tend to promote cracking must be kept out of the bath or restricted to extremely low levels. Chlorides are such materials, as noted hereinbefore. Certain materials leached from ion exchange resins tend to promote cracking. Some of these materials may be found in the salts used to make up the bath. Care must also be exercised to avoid their introduction during operation, especially if bath purification is to be attempted utilizing basic ion exchange resins. Other materials tend to widen the conditions under which it is possible to plate bright crackfree chromium, or under given conditions result in the ability to plate a greater thickness of bright crack-free chromium. [Such favorable materials are exemplified by the cations sodium, calcium and potassium.

Articles made ofany of a variety of basis metals, such as plain carbon steels, alloy steels including stainless steel, iron, copper and copper alloys, nickel and nickel alloys, zinc and zinc alloys, etc., may be plated with bright crack-free chromium. In general, the basis metal may be any metal, or an undercoat on: a metal, that can be chromium plated. I

The basis metal does affect the thickness of chromium which may be plated bright and crack-free to a certain extent. This is illustrated by the slightly greater thickness of bright crack-free chromium which can be deposited directly on steel than can be deposited directly on copper under the same process and bath conditions. The greater thickness attainable under the same conditions on the steel is attributed to the greater hardness of the basis metal and the support it is thus able to give the electrodeposit.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

We claim:

1. A process for electrodepositing bright crack-free chromium to a thickness of at least 0.00005 inch on a metal cathode comprising passing current from an anode to said cathode immersed in an aqueous chromium plating bath at a temperature between 112 F. and 140 F., said bath containing less than 0.05 g./l. of chloride ion and comprising essentially between 180 g./l. and 540 -g./l. of CrO a total catalyst ion concentration of between 1.2 g./l. and 6.3 g./l. of sulfate and silicofiuoride ions, the Ratio of 00;, concentration to total catalyst ion concentration being between :1 and 150:1, the sulfate ion content being between 20% and of the total catalyst concentration, the CIO;, concentration, Ratio and temperature having a relationship defined by the areas enclosed by the curves in Fig. l.

2. A process for electrodepositing bright crack-free chromium to a thickness of at least 0.00005 inch on a metal cathode comprising passing current from an anode to said cathode immersed in an aqueous chromium plating bath at a temperature between 117 F. and 135 F., said bath containing less than 0.02 g./l. of chloride ion and comprising essentially between 250 g./1. and 425 g./l. of CrO a total catalyst ion concentration of sulfate and silicofluoride ions to achieve a Ratio of CrO; concentration to total catalyst ion concentration between 8021 and 150:1, the sulfate ion content being between 35% and 75% of the total catalyst concentration, the CrO concentration, Ratio and temperature having a relationship defined by the areas enclosed by the curves in Fig. 2.

3. A process for electrodepositing bright crack-free chromium to a thickness of at least 0.00005 inch on a metal cathode comprising passing current from an anode to said cathode immersed in an aqueous chromium plating bath at a temperature between F. and 138 F., said bath containing less than 0.02 g./l. of chloride ion and comprising essentially between 275 g./l. and 400 g./l. of CrO a total catalyst ion concentration of sulfate and silicofluoride ions to achieve a Ratio of CrO concentration to total catalyst ion concentration between 105:1 and :1, the sulfate ion content being between 35% and 75% of the total catalyst concentration, the CrO concentration, Ratio and temperature having a relationship defined by the area ABCD of Fig. 1.

4. A process for electrodepositing bright crack-free chromium to a thickness of at least 0.00005 inch on a metal cathode comprising passing current from an anode to said cathode immersed in an aqueous chromium plating bath at a temperature between 119 F. and 140 F., said bath containing less than 0.02 g./l. of chloride ion and comprising essentially between 190 g./l. and 50S g./l. of CrO a total catalyst ion concentration of sulfate and silicofluoride ions to achieve :1 Ratio of CrO concentration to total catalyst ion concentration between 80:1 and 150:1, the sulfate ion content being between 20% and 85% of the total catalyst concentration, the C10 concentration, Ratio and temperature having a relationship defined by the area enclosed by the curve AGFE in Fig. '1.

5. The process as described in claim 1, in which the sulfate in the bath is added in the form of strontium sulfate and the silicofiuoride is added in the form of potassium silicofluoride, each in an amount suflicient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and potassium silicofiuoride, respectively, anda solublenon-catalytic potassium compound to suppress the concentration of silicofiuoride ions.

6. The process asdescribed in claim 5 in which the bath contains an addition of a soluble non-catalytic strontium compound to suppress the concentration of sulfate.

References Cited in the file of this patent UNITED STATES PATENTS 2,686,756 Stareck et al. Aug. 17, 1954 2,800,438 Stareck et al. July 23, 1957 2,800,443 Stareck et al. July 23, 1957 

1. A PROCESS FOR ELECTRODEPOSITING BRIGHT CRACK-FREE CHROMIUM TO A THICKNESS OF AT LEAST 0.00005 INCH ON A TO SAID CATHODE IMMERSED IN AN AQUEOUS CHROMIUM PLATTO SAID CATHODE IMMERSED IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATURE BETWEEN 112*F. AND 140*F., SAID BATH CONTAINING LESS THAN 0.05 G/L. OF CHLORIDE ION AND COMPRISING ESSENTIALLY BETWEEN 180 G/L AND 540 G/L. OF CRO3 A TOTAL CATALYST ION CONCENTRATION OF BETWEEN 1.2 G/L AND 6.3 G/L OF SULFATE AND SILICOFLUORIDE IONS, THE RATIO OF CRO3 CONCENTRATIOIN TO TATOL CATALYST ION CONCENTRATION BEING BETWEEN 80:1 AND 150:1 THE SULFATE ION CONTENT BEING BETWEEN 20% AND 85% OF THE TOTAL CATALYST CONCENTRATION, THE CRO3 CONCENTRATION, RATIO AND TEMPERATURE HAVING A RELATIONSHIP DEFINED BY THE AREAS ENCLOSED BY THE CURVES IN FIG.
 1. 